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SubjectRe: [patch] document flash/RAID dangers
On Wed, 26 Aug 2009, Ric Wheeler wrote:

> On 08/26/2009 08:40 AM, Theodore Tso wrote:
>> On Wed, Aug 26, 2009 at 07:58:40AM -0400, Ric Wheeler wrote:
>>>> Drive in raid 5 failed; hot spare was available (no idea about
>>>> UPS). System apparently locked up trying to talk to the failed drive,
>>>> or maybe admin just was not patient enough, so he just powercycled the
>>>> array. He lost the array.
>>>>
>>>> So while most people will not agressively powercycle the RAID array,
>>>> drive failure still provokes little tested error paths, and getting
>>>> unclean shutdown is quite easy in such case.
>>>
>>> Then what we need to document is do not power cycle an array during a
>>> rebuild, right?
>>
>> Well, the softwar raid layer could be improved so that it implements
>> scrubbing by default (i.e., have the md package install a cron job to
>> implement a periodict scrub pass automatically). The MD code could
>> also regularly check to make sure the hot spare is OK; the other
>> possibility is that hot spare, which hadn't been used in a long time,
>> had silently failed.
>
> Actually, MD does this scan already (not automatically, but you can set up a
> simple cron job to kick off a periodic "check"). It is a delicate balance to
> get the frequency of the scrubbing correct.

debian defaults to doing this once a month (first sunday of each month),
on some of my systems this scrub takes almost a week to complete.

David Lang


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